Now we come to the most popular type of feed of all: PC board feeds. Most devices use PC boards, and have the antenna(s) incorporated into the board itself. Any and all matching networks are usually going to be incorporated into the PC board, and the interconnecting feed itself is almost certainly going to be made with PC board traces as well.
Feeds with PC Board Traces
While it is possible to use miniature coax cables to interconnect antennas and matching circuitry mounted on a PC board, this is highly unlikely to be the ideal design. Feeds are usually made with PC board traces. The feed lines behave similarly to their discrete counterparts, but the main difference is that the antenna designer will need to design the feed to ensure that the impedance matches. As mentioned in our previous feed article, the ever-popular coax cable’s impedance is primarily set by the dielectric constant and thickness of the material separating the center conductor from the shield. In a PC board feed scenario, the same principles apply, only the PC board material itself and the width and spacing of the traces involved are the governing impedance-setting characteristics. It is worth adding at this point that it is certainly possible to create a twin lead with PC board traces. However, as twin lead is easily effected by external influences, a twin lead feed is highly unlikely to be suitable for the cluttered environment of a PC board. The two basic feeds most commonly used in a PC board scenario are the microstrip and the coplanar waveguide.
Microstrip
The microstrip feed is the simplest PC board feed, simply consisting of a trace over a ground plane. Though, at a glance, this may seem little better than simply connecting a loose wire between an antenna and its source of power, the microstrip in fact has a well-defined impedance. The trace of the microstrip is comparable to the center conductor of a coax cable, while the ground plane is similar to the shield of the coax. Not surprisingly, then, the impedance of a microstrip line is set first and foremost by the dielectric constant of the PC board itself and the spacing between the signal trace and its ground plane. Furthermore, the width and thickness of the feed trace also affect the impedance of a microstrip line. Though a microstrip line is rather reminiscent of a coax cable, there are some weakness associated with it. These stem from the fact that, though it behaves similarly to a coax, microstrip is not like a complete coax. Whereas the shield of a coax cable completely encircles the center conductor, totally isolating it, the microstrip is only “shielded” on one side — the ground plane side. The upshot is that the fields induced by RF flowing through the microstrip are not completely contained within the feed structure; they fringe outward. Not only does that make the microstrip line prone to external influences, including interference, but it also means that the microstrip can be rather lossy — especially at high frequencies — due to the power radiating away from it.
Coplaner Waveguide
The coplanar waveguide is an improvement on the microstrip feed, and helps add more shielding to the feed. The coplanar waveguide is essentially a microstrip with two ground traces running parallel to the feed, one on each side of the main feed trace. This extra shielding helps to contain the fields radiating from the feed wire, making the coplanar waveguide much more suitable than the microstrip feed for high-frequency and low-loss PC board applications. The extra shielding adds another layer of complexity; the spacing of the two ground traces running parallel to the main feed trace will affect the impedance of the feed. Furthermore, the ground traces will need to be connected to the groundplane with regular “vias” (i. e., conductors that electrically connect different layers of a PC board together) to ensure proper performance.
Proper Feed Design
A good PC board feed will need to avoid sharp angles as much as possible. Sharp angles will result in an impedance change in the line. A much better way to make angles is to build the feed with a rounded curve as opposed to a sharp bend. Alternatively, a sharp bend could be used if the pointed portion of the bend is shaved off to the correct depth required to adjust the impedance of the bend back to the desired value.
Another obvious point is to keep the traces a uniform width. Anywhere that the width of the PC board feed trace changes will mean an impedance mismatch. Incidentally, if done in a controlled manner, impedance transformers can actually be created by tapering or flaring a feed trace in a PC board circuit.
For proper operation of the feed, the ground plane and the parallel ground traces of a coplanar waveguide will need to be wide enough to be effective. Usually the ground plane will be no problem, as most devices have a single more-or-less solid ground plane side to the PC board. Obviously, the ground plane must be solid beneath the RF feed for proper operation.
For the ground traces of a coplanar waveguide, these traces should be wider than the gap between each trace and the main feed line. Coplanar waveguides also should be designed such that the gap between the ground traces and the feed line is smaller than the distance between the feed trace and the ground plane beneath. If this criterion is not met, the coplanar waveguide will tend to act like a microstrip in performance.
ARSI 